1. Field of the Invention
The present invention concerns a gas treatment device for purifying exhaust gas such as industrial gas, power plant gas, automobile gas and so on, or purifying gas emitted at various manufacturing plants, medical facilities and so on, by using corona discharge.
2. Description of the Related Art
An electrostatic agglomerator, an electrostatic dust collector or the like is used as exhaust gas treatment device for industrial gas, power plant gas, automobile gas and so on, and for gas emitted at various manufacturing plant, medical facilities and so on. In these equipments, by applying a high voltage between a corona electrode and a dust-collecting electrode, corona discharge is generated to charge floating fine particles such as particulate matters (PM) in gas. Then, the charged particles are attracted to the dust-collecting electrode to be agglomerated, grown, or caught or to be captured by electrostatic force.
In the electrostatic agglomerator using such corona discharge, only agglomeration and growing are performed as one component of a gas treatment system. The gas treatment system is composed so that a rear stage filter disposed at the downstream side will collect dust. In this case, by agglomerating and growing under electrostatic effect of the electrostatic agglomerator, even fine particles that would not completely filtered with a wide-mesh filter can be easily filtered in the rear stage filter.
Namely, bonding is generated among particulates captured on the surface of the dust-collecting electrode, by electrostatic force, which agglomerate and grow particulate. However, under the influence of the gas flow, the captured particles that have been grown will be separated from the surface of the collecting electrode and re-entrained.
These re-entrained particles are repeatedly charged, captured, and separated in the gas treatment device to make their particle size bigger gradually, and finally exhausted out from the device by re-entrainment. In this case, since the diameter of the particles to be exhausted has increased, a wide-mesh filter can easily catch these particles in the downstream. In short, it functions as electrostatic agglomerator.
Also, the electrostatic dust collector using this discharge agglomerates and grows particles, and collects dust. With such electrostatic dust collector, gas to be treated will be passed into a cylindrical body. A corona electrode is disposed in the substantial center of the dust-collecting electrode that is either formed by the cylindrical body, or separately provided from the cylindrical body. Corona discharge is generated in the gas and floating particle in the gas is charged, by applying a high voltage between the corona electrode and the dust-collecting electrode.
This charged particle is moved to the surface of the dust-collecting electrode and caught by this surface of the dust-collecting electrode by electrostatic power of an electric field formed between the corona electrode and the dust-collecting electrode. This captured particles is detached from the dust-collecting electrode by releasing or the like, collected and removed similarly as electric dust collector and so on, or burned and removed by heating with a heater and so on adjacent to the dust-collecting electrode. Thus, floating particle is removed from the gas.
As one example of such gas treatment device, Japan Utility Model Registration No. 3019526 proposes a cylindrical electric precipitator. In this precipitator, for removing white smoke and a small amount of pollutants in mist form in flue gas, a high voltage is applied to a high voltage discharge wire (corona electrode) formed by metal fine wires stretched in a metal tube, and floating particles such as flue gas exhausted from a factory are electrostatic adsorbed on the cylindrical electrode to remove them.
However, in electrostatic dust collection of industrial gas, automobile gas and so on, the moving speed of charged particle is, at most, on the order of several cm/s. Besides, the distance between the corona electrode and the dust-collecting electrode is usually set to several cm or more. Consequently, to obtain sufficient effects of agglomeration or dust collecting, a retention time, namely, treatment time of 1 s or more is required. As the result, there is a problem that the gas treatment device is difficult to be made compact.
The Inventors have found that turbulence acceleration is effective for increasing effect of agglomeration or dust collection on the facing surface of the dust-collecting electrode which is a surface facing the corona discharge. Based on their findings, experiments have been repeated and achieved the present invention. Namely, when turbulence is accelerated on or in the vicinity of the facing surface of the dust-collecting electrode, movement of the particles along the direction of the cross section of the flow passage that is in a vertical direction with respect to the main flow direction is accelerated. Thus, arrival speed of particles to the dust-collecting electrode is shortened, the probability of the collision among particles in the passage increases, and the time required for agglomeration and dust collection becomes shorter. Furthermore, when temperature of gas is high and temperature of the dust-collecting electrode is low, cooling of the gas in the vicinity of the dust-collecting electrode is accelerated because of increase of the heat conduction effect with the turbulence acceleration. Hence, the effect of a liquid bridge formation to particles is brought through condensing the components in the gas that are easily liquefied. Thus, effects of agglomeration, dust collection, and capture are further increased.
It should be noted that the present invention intends to accelerate gas turbulence in the vicinity of the dust-collecting electrode surface. There are some apparatuses whose structures might be similar to the present invention. As proposed in Japanese patent application Kokai publication No. 1991-173311, one of such apparatuses is an exhaust gas purifier in which a first electrode surface formed in a convex shape is provided in the center portion of a cylindrical second electrode serving as dust-collecting electrode. Another such apparatus is an exhaust gas purifier as proposed in Japanese patent application Kokai publication No. 1995-125928 in which, contrary to the Kokai publication No. 173311, a first electrode, as a dust-collecting electrode, is provided in the center portion of a cylindrical body where spiral grooves are formed on the inner wall as discharge electrode.
As proposed in Japanese patent application Kokai publication No. 1999-342350, there is also an air cleaner comprising a dust-collecting electrode made by the inner peripheral surface of the inside of a draft pipe with an annular (ring shape) cross-section, and a charge electrode (discharge electrode) made by an outer peripheral surface of the draft pipe, the charge electrode being formed with a convex portion having a sharpened summit.
However, in these exhaust gas purifiers or air cleaner, the convex shape, the spiral groove, the needle electrode array, and so on are composed to form an electric field concentration point for the purpose of making stable, uniform, and lower-voltage discharge. Besides, since such convex shape and so on are not for the object of turbulence acceleration, they are all provided on the discharge electrode and not on the dust-collecting electrode side. Consequently, it is impossible to accelerate turbulence on the dust-collecting electrode where particles are agglomerated and grown. Therefore, it is impossible to use the effect to improve agglomeration, growing, dust collection, and capture and so on by turbulence acceleration.
In addition, as proposed in Japanese patent application Kokai publication No. 2002-30921, there exists a plasma type exhaust gas purifying apparatus for decomposing NOx or the like in which an annular flow control member such as ring shape barrier or the like is provided on the inner surface of a discharge tube for biasing or guiding the exhaust gas flow in an outer electrode toward an inner electrode. In this apparatus, the quantity of exhaust gas passing through a portion of high plasma intensity in the vicinity of the inner electrode increases to improve the exhaust gas purifying efficiency as a whole.
Though the structure of this apparatus may be effective when chemical reactions of gas are increased by plasma, the improvement of dust collection performance can not be expected because the contact efficiency between gas and the inner surface of the tube decreases in the case of the dust collection such as capturing of particles on the inner surface of the tube to be a dust-collecting electrode.
In addition, with this structure, in order to guide the exhaust gas flow toward the inner electrode, the height of the ring shape barrier may be required to be the order of several tens % of the tube inner diameter. If such barrier structure is formed with a metal to be a ground potential, the sparking voltage will be strongly limited by the inner diameter of the ring. Accordingly, comparing to the case where there is no ring shape barrier, the voltage to be applicable will be remarkably lowered. Lowering the applied voltage greatly reduces the intensity of the electric field of the inner surface of the tube, resulting in remarkably lowered dust collection effect to the inner surface of the ground electrode.
Also, even if this ring shape barrier is formed with an insulating material, when the inner surface of the tube is formed with a metal at the ground potential, comparing to the case where there is no ring shape barrier, applicable voltage similarly decreases due to the electric breakdown across the surface of the insulating material. As the result, the field intensity of the tube inner surface lowers, deteriorating the dust collection effect.
Otherwise, if the ring shape barrier is formed with an insulating coating or an insulating body from the inner surface of the tube, it becomes extremely expensive and problems in terms of insulation durability will arise. When a gas treatment device is intended to collect dust, a structure for increasing the contact efficiency between gas and a surface of a dust-collecting electrode, with lowering the applicable voltage as less as possible is required.